Oxetanes are a class of organic compounds that contain a four-membered ring containing one oxygen atom. The small ring size and high degree of ring strain associated with oxetanes makes them readily polymerizable, yet difficult to synthesize. Thus, although 1,4-butanediol can be readily cyclodehydrated to give tetrahydrofuran under mild acid conditions, 1,3-propanediol when reacted under similar conditions gives no oxetane. Although polymers from oxetanes may have interesting properties, their commercialization has had only limited success because of the unavailability of highly selective, practical, economical methods for synthesizing oxetanes.
Many syntheses of oxetanes have been developed, but side reactions are common, and yields are typically less than 50 percent. In addition, the products are difficult to purify.
Searles et al. (J. Am. Chem. Soc., 79 (1957) 948) reported a synthesis of oxetanes by conversion of a 1,3-glycol to the corresponding gamma-chlorohydrin acetate, followed by ring closure with boiling aqueous caustic. The glycol is reacted with acetyl chloride and thionyl chloride prior to ring closure. Although this is one of the more commonly used methods of synthesizing oxetanes, yields are usually poor (20-40%), and substantial proportions of elimination products can be formed if the starting glycol has a hydrogen attached at the 2-carbon.
Reaction of 1,3-glycols with phosgene or dialkyl carbonates gives cyclic carbonates of the glycols, which can be cracked with bases or transition metal catalysts to give carbon dioxide and the desired oxetane. Halary et al. (Bull. Soc. Chim. Fr. (1972) 4655) showed that cyclic carbonates decompose in the presence of catalysts such as lithium chloride and silver cyanide to give oxetanes in about 30 percent yield. More recently, Bartok et al. (Acta. Chim. Hung. 114 (1983) 375) demonstrated that oxetanes could be produced by pyrolysis at 320.degree. C. of the carbonate in the presence of potassium cyanide. These methods typically give substantial amounts of side products in addition to the desired oxetane, and are therefore of limited value.
In addition, oxetanes have been synthesized from many esoteric intermediates, such as gamma-haloalkoxytributylstannanes (B. Delmond et al., J. Organomet. Chem. 47 (1973) 337), gamma-halogenated magnesium alkoxides (J. Combret et al., Bull. Soc. Chim. Fr. (1971) 3501), and gamma-hydroxyalkoxyphosphonium salts (B. Castro et al., Tetrahedron Lett. (1973) 4459. While these reactions are interesting and unusual, they are of limited practical value.
A commercially more interesting route to oxetanes is taught by Case (U.S. Pat. No. 3,006,926, and Nature, Aug. 13, 1960, p. 592). A sulfuric acid solution of a 1,3-glycol is added to boiling caustic, and the oxetane product is distilled from the reaction mixture. The yields of oxetanes are relatively low--only about 30 percent at best. In addition, numerous side reactions can occur depending upon the structure of the glycol. For example, if 2-methyl-1,3-propanediol is used in practicing this method, a substantial amount of methallyl alcohol is generated as a by-product.
Clearly, alternative methods for synthesizing oxetanes are needed. Methods involving simple, inexpensive reagents are required so that the synthesis can be carried out on a commercial scale. Methods that give the desired oxetane products in high yield and in the absence of rearrangement or elimination products are especially needed.